Congratulations, it sounds like you've got yourself a really nice 55. From your description (and there are variables) the car would cost $200K-$250K to be built in a very high end shop. To sell it afterwards you'd be lucky to get much north of $125K. If he truly paid $600K to any shop to get it to where its at, I'd think they'd want to keep working on it forever as its been a unbelievably huge cash cow for them. My guess again from your description is you're $50K or so from having it done and it'll bring much more completed than selling it as is. Good luck with it, pictures would be helpful to give you better more detailed advise.